Читать книгу Strength Of Beams, Floor And Roofs - Including Directions For Designing And Detailing Roof Trusses, With Criticism Of Various Forms Of Timber Construction - Frank E. Kidder - Страница 13

Rule 15.—To find the safe load for a cylindrical beam, as a log, first find the strength of a square beam (loaded in the same way) whose sides are equal to the diameter of the round beam, and divide the answer by 1.7. If the beam tapers slightly, as in the case of the trunk of a tree, measure the diameter at the center of the span.

Example X.—What is the safe center load for a spruce pole 12 inches in diameter at the center and with a span of 16 feet?

Answer.—By Rule 2 we find that the strength of a spruce beam, 12 inches square and 16 feet span, equals 12 × 12 × 12 × 70 divided by 16 = 7560 pounds. Dividing this by 1.7 we have 4447 pounds for the answer.

To determine the diameter of a cylindrical beam to support a given load at the center.

Rule 16.—Multiply the span by the load and the product by 1.7 and divide by the value of A. The cube root of the result will be the answer.

Example XI.—Find the diameter of a round spruce pole of 16 feet span to support a center load of 4447 pounds.

Answer.—4447 × 16 × 1.7 = 120,958.4. Dividing this by 70, the value of A, we have 1728. The cube root of 1728 is 12, the required diameter of the pole.

If the load is distributed, divide it by 2 and then proceed by the above rule.